The present invention relates to a brake power servo booster and particularly to a brake power servo booster in which the loss stroke of an input shaft at the beginning of the operation is reduced. A brake power servo booster of this type is arranged such that when a power piston is retracted, a wide flow path area is maintained in a valve mechanism to enable the retreating operation of the power piston to be performed quickly. During the non-actuated period of the brake power servo booster, the flow path area is made small or zero so that the flow path of the above-mentioned valve mechanism can be immediately switched over at the advancing operation of the input shaft upon the next depression of a brake pedal. Therefore, in the present invention the loss stroke of the input shaft, required before the input shaft can be advanced, can also be made small in comparison with the conventional brake power servo booster in which the above-mentioned flow path is left wide even in the non-actuated state of the brake power servo booster.
Conventionally, in order to obtain the function mentioned above in a brake power servo booster in which a master cylinder is connected to a front shell and a rear shell is connected to a car body, a stopper has been provided in the rear shell for limiting the retreat position of an input shaft so as to prevent the free retreat of the input shaft relative to a valve mechanism of the input shaft only in the non-actuated state of the brake power servo booster, thereby decreasing the loss stroke. Alternatively, a key member may be provided for preventing a valve plunger, constituting a valve mechanism, from coming off from a valve body. The key member may be slidably provided in the valve body so that the key member is made to contact with the inner surface of a rear shell in the non-actuated state to thereby prevent the free retreat of the key member relative to the valve body, and therefore prevent the free retreat of an input shaft which is interlocked with the key member through a valve plunger, so as to decrease the above-mentioned loss stroke.
In the former arrangement, however, although there is an advantage in that the stopper member, which is made to contact with the input shaft, is outside the shell so that the retreat position of the input shaft can be easily adjusted, there is also a disadvantage, in that because the stopper member is required to make contact with the input shaft in a manner such that the stopper member covers the valve body, which projects relatively far out of the rear shell, the stopper member is required to be highly rigid, resulting in an increase in weight as well as in the overall length of the brake servo booster. In the latter arrangement, in contrast to the former, although there is an advantage in that the increase in weight and in overall length can be kept to a minimum since only the improvement in the key portion is required, there is a disadvantage in that the adjustment of the retreat position of the input shaft becomes difficult because the key portion is located within the shell.